It’s March, and although we still have a lot of snow on the ground here in the Northeast, soon the mosquitoes will be buzzing. In addition to making us bumpy and itchy, mosquitoes carry diseases. For dogs and cats, the threat is heartworm, which is now found in all fifty states. The mosquito carries the immature form of the worm, which is transferred into a dog or cat’s bloodstream during the bite. It takes 6-7 months after the mosquito bites a dog for the worms to mature to adults, at which point they start producing the infective form called microfilariae. By 3-4 months after the mosquito bite, immature worms have reached the lungs, and they will eventually be forced into the right ventricle of the heart.
Clearly, this is a bad situation for any animal, to have worms in its lungs and heart. Dogs are the definitive host for heartworm and act as the main reservoir for the disease. Cats are susceptible but much more resistant to infection than dogs. What this means is that dogs tend to develop a higher number of worms than cats, but cats are much smaller critters so even a few worms can cause serious problems. Any way you look at it, having worms in one’s lungs and heart is a raw deal.
When you hear your vet and his/her staff going on and on about heartworm prevention, this is why. Prevention is truly worth an ounce of cure. First of all, untreated heartworm disease can lead to congestive heart failure. Secondly, the treatment for heartworms involves injecting an arsenic-based drug deep into the muscle, after which the worms die and float off into the bloodstream. The serious consequences of treatment include inflammation of blood vessels and worst case, pulmonary thromboembolism. Dogs undergoing treatment (which takes several months typically) are required to have strict exercise restriction to reduce the chance of complications from emboli. Prevention involves giving your dog a monthly heartworm preventative, in the form of a chewable pill or a topical liquid. Certainly much easier, safer, and less expensive than having your dog undergo treatment for heartworms.
Here’s the new stuff for this year.
The American Heartworm Society (AHS) now recommends the following:
- Yearly antigen and microfilariae testing. This means a blood test for both adult and immature heartworms.
- Year-round administration of heartworm preventive medications (the chewable tablet or topical liquid mentioned above).
Why test every year and give preventative year-round? First, there is no medication that is 100% effective, 100% of the time. We also don’t know how many times our dogs spit out the tablet we give them behind the couch. Finally, the AHS is on record this year stating there have been some documented pockets of resistance to the commonly used heartworm preventatives. Data about resistant strains has shown that extended treatment after exposure is needed to prevent infection. Thus, treatment year-round is suggested for maximal effectiveness.
In my next post I’ll write in more detail about the various heartworm preventatives, giving some pro’s and con’s for each. I sincerely hope none of you have had to deal with treating heartworms in your pet. If you have, would you share your pet’s story with me?